The invention relates to a screw press for separating liquids from solid-liquid mixtures, especially pulp suspensions, which has a casing provided with liquid passages, the casing especially divided into segments, and having a screw mounted on a shaft, which is preferably hollow, rotating inside the casing, and including a suspension feed area.
When dewatering suspensions with low consistencies, it is only possible to achieve good dewatering performance in the inlet area of a screw press at very low pressures. At higher pressure, the stock, particularly a pulp suspension, is pushed onto the openings in the screen from the inside and thus plugging openings and impeding the dewatering action. After the first rise in pressure, the filtrate no longer flows out of all the holes in the screen, but only out of those that have just been cleared by the rotating screw flight moving past. If the pressure rises further, the screw flight cannot clean the screen at all because of the required gap between flight and screen unless there are already drier pulp fibers upstream of the flight which bridge the gap. The clogged screen surface is then lost for dewatering purposes. In the pulp distribution boxes already known and which are located at the beginning of the screw, the screen basket starts to clog as the pressure rises from the beginning of the screw towards its end.